Magnetic resonance imaging (MRI) of the brain can aid physicians in distinguishing semantic dementia from Alzheimer’s disease, two neurodegenerative disorders that are hard to differentiate in their early stages. These findings, which appear in the April 2001 issue of the Annals of Neurology, the research journal of the American Neurological Association, will allow neurologists to counsel patients more effectively and determine which patients are suitable for treatment with anti-dementia therapies.
“In the longer term, accuracy of diagnosis will assume a greater clinical importance with the development of new therapeutic treatments for specific diseases that cause dementia,” said senior author Nick C. Fox, M.D., a neurologist at the National Hospital for Neurology and Neurosurgery in London.
Unusual forgetfulness in middle and older age is usually ascribed to Alzheimer’s disease, even by physicians. However, there are a number of other disorders, including strokes and other dementias, that have similar manifestations. Semantic dementia and Alzheimer’s disease are particularly difficult to differentiate by traditional clinical tests of cognitive function.
Semantic dementia is characterized by the inability to match certain words with their images or meanings (semantic memory). However, patients with this disorder retain the ability to speak quite fluently, as well as the ability to remember day-to-day events (episodic memory). Alzheimer’s disease is primarily characterized by a failure of episodic memory, but semantic memory is affected during later stages of the disease.
For the current study, Fox and his colleagues analyzed the MRI brain scans of 30 subjects–10 patients with semantic dementia, 10 patients with Alzheimer’s disease, and 10 subjects with normal cognitive function. They found significant differences in the patterns of atrophy in the temporal lobes of the brain, areas known to play an important role in memory.
In patients with semantic dementia the loss of brain tissue was mostly confined to the left side of the brain and particularly to the front portion of the left temporal lobe. In patients with Alzheimer’s disease, the degree of atrophy was equivalent on both sides of the brain, with no evidence to suggest greater atrophy in the front portion, compared to the back portion, of the temporal lobes.
The authors believe that their findings may have an impact on physicians’ ability to counsel their patients more effectively and to suggest more appropriate courses of treatment. “Counseling of patients and families on these disorders is our responsibility, and that counseling varies depending upon our diagnoses,” said neurologist Ronald C. Petersen, Ph.D., M.D., of the Mayo Clinic in Rochester, Minnesota, who wrote an editorial accompanying the article. “For example, we know a great deal about the inheritance of Alzheimer’s disease, but we cannot generalize that information to other dementias. Some of them have known inheritance patterns, which can be quite different from Alzheimer’s disease, and this concept can have huge implications for families.”
Petersen does not think that MRI analysis on its own will be sufficient in distinguishing between disorders such as semantic dementia and Alzheimer’s disease. He anticipates that neurologists will need to consolidate traditionally acquired clinical information with clues from newer radiologic, genetic, and laboratory tests to assign accurate diagnoses for brain disorders with overlapping symptoms.